The use of optical fibers in communications has grown significantly over the past few years. It is anticipated that its use will reach into the residential loop distribution system in the near future. For now, loop distribution cables which include insulated metallic conductors continue to be installed.
Optical fiber has found widespread application in longhaul transmission such as, for example, between cities, and in trunk service, for example, between telephone central offices or switching centers. It is readily apparent that optical fiber can be used not only in longhaul and trunk applications but also in local area networks, service or drop applications, and in indoor cable. As a result, end to end optical communication is fast becoming a reality.
Nevertheless, operating telephone companies have expressed a desire to install cables which include metallic conductors as well as optical fiber capability. Notwithstanding the rapidly escalating use of optical fiber, there remains a need for metallic conductors such as copper conductors in order to modulate and demodulate digital or analog optical signals. Metallic conductors may be sought after to provide power or may be used for other low voltage signaling purposes. Furthermore, there may be a need for both optical fiber and metallic conductors at the same end use point. One such application may be at office workstations in a computerized facility. Another may be to provide addditional transmission media for metallic circuitry not yet fully utilized.
Such a need translates into a need of a hybrid cable which is intended to refer to a cable which offers the capability for both optical fiber and metallic transmission. A course of action of early placement of optical fiber capability along with metallic conductors in aerial or buried installations to customers' premises will facilitate the later transition from a metallic to an optical fiber operating system. By capability is meant the installation of facilities which at a later date facilitate the installation of optical fiber of a cost greatly reduced over that of a complete installation. Obviously, the first cost of installing optical fiber capability to customers' premises is minimized by such an approach because the capability of providing optical fiber transmission can be installed simultaneously with the installation of metallic transmission media for essentially the same cost as either separately. With such a cable, optical fiber or optical fiber capability can be provided to customers' premises awaiting the arrival of the optical fiber network and development of associated hardware and electronics.
Such cables initially having optical fiber capability may be placed by the same methods and apparatus as are used for all-copper cables. Accordingly, the optical fiber portion thereof must be robust enough to withstand plowing and trenching or aerial stringing of a host structure and to be capable of survival outside the host structure in a separate run to an optical fiber storage or termination point.
With such a cable structure in place, service will evolve from the metallic pairs to the optical fibers. Simple telephone service can begin immediately over a metallic pair of conductors. Other metallic conductor pairs of the distribution and service cable can serve as secondary lines or alarm circuits. Initially, the optical fiber unit may be used to provide cable television or be retained for later use.
At a later date, more sophisticated offerings such as, for example, electronic newspapers and mail, catalogs and shopping, banking and business activities and data and computer functions which require increased bandwidth and customer interaction provided by optical fiber transmission may be served through a remote terminal. For these applications, metallic conductor pairs may provide power to on-premise electronics or serve as control circuits. Still later, all offerings may be provided over the optical fiber media, but power still will have to be provided for on-site electronics by the power or telephone operating company. Providing power from a central office source through these structures should result in reliable telecommunications during power outages. Also, the copper conductor pairs may have other uses such as circuit maintenance, for example.
The sought-after cable should have desired properties. For example, it should have a relatively high tensile and compressive loading capability, a relatively low minimum bend radius, stiffness in order to insure that the optical fiber unit remains as straight as possible to minimize bend losses, an operating temperature range of about -40.degree. to +160.degree. F. and low cost. The cable should be able to withstand repeated impact during installation. Also, the structure must not be affected adversely by cable filling compounds. The cable must be water-resistant to prevent degradation of transmission or damage due to water-induced crack propagation or freezing. In those instances when it will connect to customers' premises, the cable must be capable of being made flame retardant. Also, the sought after cable should be relatively easy to manufacture and to connectorize despite the capability for different kinds of transmission media.
Hybrid cables are known to the prior art. For example, described in F. J. Mullin-W. S. Reed U.S. Pat. No. 4,852,965 which issued on Aug. 1, 1989 is a hybrid cable which includes a reinforced optical fiber unit. The unit includes an optical fiber or fibers enclosed by at least three impregnated fiberglass members and a jacket. The unit is included along with metallic conductors in a core tube enclosed in a plastic jacket.
Also important is access to the transmission media. In some prior art composite cables, optical fibers are disposed in a central portion thereof with copper conductors arranged thereabout. See, for example, U.S. Pat. No. 4,552,432. As a result, in order to access the optical fiber, a craftsperson needs to work through the copper conductors. Further, from a manufacturing standpoint, sometime conflicting sheath design constraints for copper and fiber technologies as well as the configuration required for various fiber and/or conductor counts increases greatly the cable complexity and cost.
In the alternative, an initially installed cable may include the metallic transmission media and a duct in which optical fibers are to be installed at a future date. When a duct for receiving optical fiber in the future is installed, the major port, ion of the construction is accomplished initially. At some future date, optical fiber is caused to be moved into the existing duct with minimum further construction activity.
Seemingly, the prior art is devoid of such a cable which provides both metallic and optical fiber capability along with desired properties such as the capability of easily accessing either the metallic conductors or the optical fiber or both. The sought-after cable will fill a need in the marketplace as services to the home are expanded.